The invention is related to temperature sensing devices and particularly to temperature sensor probes with reduced time response to changes in the temperature of a fluid being sensed. The invention also encompasses methods for reducing the time response of temperature sensor probes.
Temperature sensing devices include generally a temperature sensor probe comprising a sensing element housed in a protective sheath or housing and a suitable measuring apparatus. The probe is adapted to be inserted into a medium to be sensed so that the probe, including the sensing element, is heated or cooled to the temperature of the medium. The temperature sensing element exhibits some characteristic that varies in a single-valued fashion with its temperature and this characteristic is measured by the measuring apparatus to provide an indication of the sensing element temperature and thus the temperature of the medium in which the element is immersed. The protective housing serves to isolate the sensing element from the corrosive effects of the medium being sensed and also serves to provide strength and durability that the sensing element itself may lack.
Electrically conductive wire coils are commonly used as temperature sensing elements in temperature sensor probes. The electrical resistance exhibited by the coil varies with the temperature of the coil and thus the measurement of the coil's resistance with appropriate resistance measuring circuitry provides an indication of the coil's temperature. The temperature sensing coil is commonly housed in a cylindrical protective sheath or housing, the coil being arranged inside the cylindrical housing in order to facilitate maximum heat transfer between the housing and the coil.
One critical factor in the performance of a temperature sensor probe is time response. The time response of a temperature probe is the time it requires for its temperature sensing element to reach a temperature representing a certain percentage of the final value of a temperature step input. That is, time response is the time it takes for the probe's temperature sensing element to reach a certain percentage of the temperature of the medium being sensed. The longer it takes a sensor probe and its temperature sensing element to reach the temperature of the medium being sensed, the worse the time response of the sensor probe.
One manner in which time response may be improved is by reducing the mass of the probe. U.S. Pat. No. 3,237,139 to Werner, U.S. Pat. No. 2,588,014 to Knudson, and U.S. Pat. No. 3,436,713 to DiNoia disclose various methods for reducing time response through reducing the mass of the temperature sensor probe.
Although time response may be improved by reducing probe mass, the extent to which the mass of the temperature probe may be reduced is limited by the strength of the probe required for a particular temperature sensing application. Many temperature sensing applications require very durable and robust probes. For example, sensing the temperature of a fluid flowing in a high velocity stream requires a sensor probe capable of withstanding the force exerted by the fluid.
Another approach to improving the time response of a temperature sensor probe is to increase the convective heat transfer coefficient of the probe. The higher the convective heat transfer coefficient of the probe, the more quickly heat is transferred between the medium whose temperature is being sensed and the temperature sensing element of the probe, and thus the quicker the time response of the probe.
Since temperature sensing coils are commonly wound into a cylindrical shape, temperature sensor probes are commonly also cylindrical. Also, the cylindrical probes, when used to sense a flowing fluid, are generally positioned extending either transversely or axially to the direction of fluid flow. However, the flow pattern of a fluid as it flows past a plain cylindrical probe in a direction perpendicular or parallel to the probe's longitudinal axis produces an unfavorable convection heat transfer coefficient for the probe. Where the flow is perpendicular to the probe, the unfavorable heat transfer coefficient results from the separation of fluid flow from the surface of the probe and the formation or shedding of large vortices on the downstream side of the probe in a phenomenon known as Karman Vortex Street. Where the flow is parallel to the cylindrical probe, that is, where the probe's longitudinal axis is aligned with the direction of flow, the fluid flow separates from the probe surface in a recirculating zone extending down a portion of the probe's length.
U.S. Pat. No. 4,467,134 to Pustell teaches one method for increasing the convective heat transfer coefficient of a plain cylindrical sensor probe to reduce time response and thereby improve probe performance in sensing the temperature of a fluid flowing past the probe. Pustell discloses a probe having an elongated cylindrical housing adapted to extend in the common position transverse to the direction of fluid flow. According to Pustell's invention, the probe is mounted coaxially within a cylindrical shroud having perforations at particular orientations with respect to the direction of fluid flow. The perforated shroud serves to increase the velocity of the fluid flowing over the cylindrical probe and the increased fluid velocity reduces flow separation from the probe thereby increasing the heat transfer between the fluid and the probe. By increasing the rate of heat transfer between the fluid and the probe, the time response of the probe is decreased.
Although the perforated shroud disclosed by Pustell does decrease the time response of the temperature sensor probe, there are several problems associated with the use of such shrouds. First, the separate shroud increases probe production costs, both with regard to materials and in the manufacturing process. Also, the perforations in the shroud must be arranged in a particular orientation with respect to the direction of fluid flow to create the desired velocity increase. Substantial deviation from the particular orientation not only defeats any benefit gained from the shroud, but may even increase the probe's time response.
The model MA-1 sensor probe manufactured by Rosemount Engineering, Inc. illustrates a method for improving the time response of a temperature sensor probe aligned axially or parallel to the direction of fluid flow. This probe includes two concentric platinum tubes with a sensor element coiled around the inner tube. The probe allows fluid being sensed to flow through the inner tube where no vortices develop and the flow remains substantially in contact with the inner tube wall, thereby increasing the heat transfer coefficient. However, the coaxial, spaced apart tube configuration substantially increases probe production costs.